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Normalization and Codd's 12 Criteria

The following is a complete list of Codd's 12 Criteria, which serve as a guideline to the rules for database relationships:
  1. All information in a relational database is represented explicitly at the logical level in exactly one way: by values in tables.
  2. Each and every datum (individual value) in a relational database is guaranteed to be logically accessible by resorting to a combination of table name, primary-key value, and column name.
  3. Null values (distinct from the empty character string or a string of blank characters or any other number) represent missing information in the RDBMS in a systematic way, independent of data type.
  4. The database description is represented at the logical level in the same way as ordinary data, so that authorized users can apply the same relational language to its interrogation as they apply to regular data.
  5. A relational system may support several languages and various modes of terminal use (for example, fill-in-the-blanks mode). However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings and that is comprehensive in supporting all of the following items:
    • Data definition
    • View definition
    • Data manipulation (interactive and by program)
    • Integrity constraints
    • Transaction boundaries (begin, commit, and rollback)

  1. All views that are theoretically updateable are also updateable by the system.
  2. The capability of handling a base relation or a derived relation as a single operand applies not only to the retrieval of data but also to the insertion, update, and deletion of data.
  3. Applications programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representation or access methods.
  4. Application programs and terminal activities remain logically unimpaired when information-preserving changes of any kind that theoretically permit unimpairment are made to the base tables.
  5. Integrity constraints specific to a particular relational database must be definable in the relational data sublanguage and storable in the catalog, not in the application programs. A minimum of the following two integrity constraints must be supported:
  1. Entity integrity: No component of a primary key is allowed to be null.
  2. Referential integrity: For each distinct nonnull foreign key value in a relational database, there must exist a matching primary-key value from the same domain.
    • An RDBMS has distribution independence: A characteristic of certain Relational Database Managment Systems(RDBMS) in which storing parts of a database on more than one computer does not affect the RDBMS's ability to work with the database.
    • If a relational system has a low-level (single-record-at-a-time) language, that low-level language cannot be used to subvert or bypass the integrity rules or constraints expressed in the higher level relational language (multiple-records-at-a-time).